System for supplying secondary air in the exhaust system of an internal combustion engine

ABSTRACT

A reciprocating piston engine ( 2 ) includes an exhaust system comprising an exhaust pipe ( 8 ) which communicates with a silencer ( 10 ), whose upstream portion ( 11 ) is divergent in the direction of gas flow through it, an oxidising catalyst ( 12 ) and an air supply pipe ( 14 ) communicating with the exhaust pipe at a position upstream of the catalyst and silencer. The air supply pipe ( 14 ) includes a Reed valve ( 16 ) which is adapted to open under a pressure differential to permit air to flow into the exhaust pipe. In order to maximise the air flow into the exhaust pipe the effect of pressure pulses within the exhaust pipe is utilised and for this purpose 3L 2 −(2L 1 +L 2 ) is equal to ±0.25 to 0.5 m, wherein is the distance from the junction of the exhaust pipe ( 8 ) measured from a first intersection point at which the axes of the exhaust pipe ( 8 ) and the air supply pipe ( 14 ) intersect, to a point midway along the length in the flow direction of the said upstream portion ( 11 ) of the silencer ( 10 ) and L 2  is the length of the air supply pipe ( 14 ) from the Reed valve ( 16 ) to the said first intersection point.

[0001] The present invention relates to reciprocating engines of fourstroke and more particularly two stroke type and is concerned with thattype of such engine which includes an exhaust system, the exhaust systemcomprising an exhaust pipe which communicates with a silencer, whoseupstream portion is divergent in the direction of gas flow through it,one or more oxidising catalysts and an air supply pipe communicatingwith the exhaust pipe at a position upstream of the catalyst andsilencer.

[0002] The exhaust gases of two stroke engines are usually rich inunburnt hydrocarbons, that is to say oil and gasoline, and carbonmonoxide, as a result of incomplete combustion and of the fact thatpurging of the combustion space is commonly performed with anair/gasoline mixture. In order to prevent excessive emissions of suchunburnt hydrocarbons to the atmosphere it is common to provide anoxidising catalyst in the exhaust system whose purpose is to oxidise thehydrocarbons and carbon monoxide to carbon dioxide and water.

[0003] However, there is generally insufficient oxygen in the exhaustgas flow to completely oxidise all the hydrocarbons and carbon monoxide.Furthermore, the substantial amount of such substances that are oxidisedresult in the catalyst reaching a very high temperature and thus in itsbeing progressively degraded and its service life shortened.

[0004] It is therefore known to provide an air supply pipe communicatingwith the exhaust pipe at a point upstream of the catalyst in order bothto provide additional oxygen for oxidising purposes and to cool thecatalyst. U.S. Pat. No. 5,902,971 discloses such an engine in which theair supply pipe is connected to a diaphragm pump, which is operated bythe pressure pulses in the crankcase and supplies the necessary air tothe exhaust system. However, the diaphragm pump adds not inconsiderablyto the weight, cost and complexity of the engine. U.S. Pat. No.5,887,424 discloses such an engine in which air is induced into thesilencer by the ejector or entrainment effect. This adds a considerabledegree of complexity to the silencer and means that the exhaust systemhas to be designed to maximise the entrainment of air rather than engineefficiency.

[0005] Accordingly it is an object of the present invention to provide atwo stroke engine with means for supplying air into the exhaust systemwhich is cheap, light and effective, and in particular does not use apump or ejector, and permits the exhaust system to be designed with aview to maximising engine efficiency.

[0006] In accordance with the present invention, in an engine of thetype referred to above the air supply pipe includes a Reed valve adaptedto open under a pressure differential to permit air to flow into theexhaust pipe and the difference between L₂ and (2L₁+L₂) is between 0.25and 0.5 m, preferably 0.3 and 0.4 m and particularly preferably 0.35 to0.4 m, wherein L₁ is the distance from the junction of the exhaust pipemeasured from a first intersection point at which the axes of theexhaust pipe and the air supply pipe intersect, to a point midway alongthe length in the flow direction of the said upstream divergent portionof the silencer and L₂ is the length of the air supply pipe from theReed valve to the said first intersection point.

[0007] The invention is based on the recognition that there are wildlyvarying pressures within the exhaust system and that the pressure wavecaused when the interior of the or each cylinder of the engine initiallycommunicates with the exhaust system at the beginning of expulsion ofthe exhaust gas can be reflected and cause the pressure locally withinthe exhaust system to fall briefly to sub-atmospheric values of e.g. amaximum of −1000 mbar gauge or more typically down to −400 mbar gauge,e.g. −100 to −300 mbar gauge. Such sub-atmospheric pressures can besufficient to open a Reed valve and cause a small amount of air to flowthrough it. Accordingly the air supply pipe is provided with a Reedvalve which is caused to open periodically by reduced pressure pulseswhich act on it.

[0008] However, it has been found that the provision of a Reed valvealone does not necessarily result in sufficient air being induced intothe exhaust system and that the amount of air that is induced isdependent on the relationship between certain dimensions of the exhaustsystem. This will be explained in more detail below.

[0009] In use, as exhaust gas starts to flow into the exhaust pipe ineach cycle of the or each cylinder of the engine, a positive pressurewave passes down the exhaust pipe at substantially the speed of sound.When this wave reaches the junction with the air supply pipe itpropagates both along the exhaust pipe and along the air supply pipewaves.

[0010] The first positive wave that continues towards the silencer isprogressively reflected back as it reaches the conically divergingportion at the upstream end of the silencer, but in the form of anegative pressure wave, due to the fact that the cross-sectional area ofthe silencer is inherently larger than that of the exhaust pipe. Theplane from which the positive wave is reflected is therefore effectivelyhalf way along the conically diverging portion in the flow direction.When this negative pressure wave meets the junction with the air supplypipe it moves up the air supply pipe. When this negative pressure wave,which will be referred to as the first negative pressure wave, reachesthe Reed valve it causes it to open for a short period of time, therebyadmitting air into the exhaust pipe.

[0011] The second positive wave that continues towards the Reed valve isreflected back at the Reed valve in the form of a positive wave towardsthe exhaust pipe. When it reaches the exhaust pipe it expands and isreflected back towards the Reed valve, but in the form of a negativewave. When this further negative wave, which will be referred to as thesecond negative pressure wave, reaches the Reed valve it causes it toopen and admit air. If the phasing of the two negative pressure waves issuch that the two waves arrive at the Reed valve substantiallysimultaneously or substantially overlapping with one another at the Reedvalve, it is found that not only is a relatively small volume of airinduced but also the force applied to the Reed valve may be sufficientto break it. If the two waves do not overlap at all at the Reed valve,the valve is caused to open twice for two very short periods of time butmuch of the energy of the waves is consumed by opening the valve andrelatively little air is caused to flow into the air supply pipe.However, if the phasing of the two waves is such that they overlap veryslightly at the Reed valve, the valve will be held open for a longerperiod of time, i.e. the sum of the duration of the two negative waves,and a sufficient volume of air is induced to achieve the desiredbeneficial effect. The relative phasing of the two negative pressurewaves is determined by the distance which they have travelled, namely3L₂ and 2L₁+L₂, respectively. If the time taken to travel the differencebetween those two distances at the speed of sound is slightly less thanthe duration of one of the waves, then the waves will overlap slightlyat the Reed valve. Since the speed of sound will vary with temperatureand the different pipes are at different temperatures, the distancesreferred to above should be corrected for temperature, namely typically30° C. in the air supply pipe and 500° C. in the exhaust pipe. It isimmaterial whether the first negative pressure wave arrives at the Reedvalve before or after the second negative pressure wave and this is whyone length subtracted from the other may result in a positive ornegative value.

[0012] If the negative pressure wave reflected back from the divergentportion of the silencer reaches the exhaust valve or port as it opens,this will promote the efficient discharge of exhaust gas from thecylinder thereby increasing delivery ratio and will thus increase thepower output from the engine. Whilst this is desirable for someapplications, the invention is particularly applicable to small twostroke engines of the type which are fitted to small motor scooters ormopeds. Some countries have legislation prohibiting such engines fromproducing more than a prescribed power output. It is therefore commonfor such engines to be provided with a blind resonator pipe, whosediameter is less than that of the exhaust pipe and which communicateswith the exhaust pipe. This resonator pipe communicates with the exhaustpipe at a position whose distance from the mid-point of the divergingportion of the silencer is substantially equal to its length. In use,the positive pressure wave caused by the opening of the exhaust valve isagain reflected back from the silencer in the form of a negative wavewhich travels back towards the exhaust valve. However, the positive wavealso travels up the resonator pipe and is reflected back from its closedend, still in the form of a positive pressure wave. This wave re-entersthe exhaust pipe and also travels towards the exhaust port. Theresonator pipe is positioned and dimensioned so that the aforementionedpositive and negative pressure waves arrive at the exhaust port at thesame time, whereby the positive pressure wave counteracts the effect ofthe negative pressure wave and there is no enhancement of the poweroutput of the engine.

[0013] It will, however, be appreciated that the positive pressure wavethat is reflected back down the resonator pipe and passes along theexhaust pipe towards the exhaust valve is also reflected back into theresonator pipe for a second time, though now in the form of a negativepressure wave due to the fact that the resonator pipe is smaller thanthe exhaust pipe. This negative pressure wave is reflected back from theclosed end of the resonator pipe and then passes into the exhaust pipe.It then moves towards the exhaust port and also towards the Reed valve.The negative pressure wave reflected back from the silencer will arriveat the Reed valve but will not have the beneficial effect of opening theReed valve because its effect is neutralised by the positive pressurewave from the resonator pipe discussed above, which arrives at the Reedvalve at substantially the same time. If, however, the negative pressurewave from the resonator pipe discussed above arrives at the Reed valveat a time at which it overlaps slightly by the second negative pressurewave described above in connection with the first embodiment with noresonator pipe then the same beneficial effect may be obtained as inthat previous embodiment. In order to obtain this effect the value of3L₂−(L₂+2L₃+4L₄) should be equal to +0.25 m to +0.45 m or −0.45 m to−0.6 m, or more preferably +0.15 m to +0.35 m or −0.35 m to −0.5 m,wherein L₂ is the length of the air supply pipe from the Reed valve to afirst intersection point at which the axes of the exhaust pipe and theair supply pipe intersect, L₃ is the distance between the firstintersection point and a second intersection point, at which the axes ofthe resonator pipe and the exhaust pipe intersect and L₄ is the lengthof the resonator pipe from its closed end to the second intersectionpoint. The distances must of course again be corrected for thetemperature at which they actually operate. There are again two possibleranges into which the distance covered by one pressure wave subtractedfrom the distance covered by the other pressure wave may fall because itagain does not matter in which order the two pressure waves arrive atthe Reed valve.

[0014] Further features and details of the invention will be apparentfrom the following description of two specific embodiments which isgiven by way of example only with reference to the accompanyingdrawings, in which:

[0015]FIG. 1 is a highly schematic view of the relevant portions of atwo stroke engine in accordance with the invention; and

[0016]FIG. 2 is a similar view of an alternative construction of a twostroke engine in accordance with the invention.

[0017] The engine includes a crankcase, a cylinder barrel or block and acylinder head which form no part of the present invention and aregenerally designated 2 in FIG. 1. The cylinder block defines one or morecylinders in which respective pistons are mounted to reciprocate. Thepistons are connected by respective connecting rods to a crankshaft.

[0018] Connected to the or each exhaust port of the engine is an exhaustsystem including an exhaust pipe 8 whose downstream end is connected toa silencer 10. The silencer 10 has an upstream conically divergentportion 11 followed in this case by a cylindrical portion and then aconically convergent portion. The silencer communicates with theatmosphere into which the exhaust gases from the engine are discharged.The exhaust system also includes an oxidising catalyst 12, which in thiscase is situated within the silencer 10. The purpose of the oxidisingcatalyst is to catalyse the conversion of unburnt hydrocarbons and COinto water and CO₂. Communicating with the exhaust pipe 8 at a positionbetween the engine block/cylinder head and the silencer 10 is an airsupply pipe 14, the diameter of which is less than the diameter of theexhaust pipe. The air supply pipe 14 includes a passive Reed valve 16which is normally closed but which will open when the pressure appliedto it on the exhaust pipe side is less than that on the other side. TheReed valve 16 comprises a valve seat 15 and a valve flap 17. The Reedvalve communicates with the atmosphere via an air supply pipe 14 whichincludes an air filter 4.

[0019] The generation, reflection and timing of the pressure waveswithin the exhaust system has already been explained above. However,briefly, the distance L₁ is the distance between a point mid-way alongthe length in the flow direction of the divergent portion 11 of thesilencer 10 and the junction of the exhaust pipe 8 with the air supplypipe 14, measured from the point at which their axes intersect. When theexhaust port opens a positive pressure wave moves down the exhaust pipe8. When it reaches the air supply pipe 14 it passes along the air supplypipe as well as continuing along the exhaust pipe. The first positivewave moves up the air supply pipe, is reflected back at the Reed valve,moves back to the exhaust pipe and then reflected back into the airsupply pipe, but in the form of a negative wave which ultimately arrivesat the Reed valve. The second positive wave moves along the exhaust pipeand is reflected back at the silencer, effectively at a plane which ishalf way along the length of the diverging portion 11, in the form ofnegative wave. The negative wave moves back along the exhaust pipe andsplits into two at the air supply pipe. One of these negative waves thenarrives at the Reed valve. The distances travelled by the two negativewaves which arrive at the Reed valve are related by the formula givenabove so that they arrive at the Reed valve at times such that theyoverlap slightly. The Reed valve is thus held open for one single andrelatively long period of time for each time that the exhaust valveopens and this period of time is sufficiently long to admit a sufficientvolume of air to cool the catalyst adequately and to result in thecombustion of the catalyst of substantially all the unburnt hydrocarbonsand carbon monoxide in the exhaust gas.

[0020] In the alternative construction illustrated in FIG. 2 the exhaustpipe additionally communicates with a resonator tube 20 which isprovided for the reason explained above. The length of the resonatortube is substantially equal to its distance from the mid-point of thediverging portion of the silencer and its diameter is less than that ofthe exhaust pipe.

[0021] When the exhaust valve opens a positive wave moves along theexhaust pipe and splits into two at the junction with the air supplypipe. The first positive wave moves three times along the air supplypipe and ultimately arrives at the Reed valve in the form of a negativewave, precisely as in the first embodiment. The second positive wavecontinues along the exhaust pipe until it reaches the resonator pipe andthen splits into two positive waves, which will be referred to as thefirst and second positive waves. The first positive wave moves along theexhaust pipe and is then reflected back from the silencer in the form ofa negative wave. The second positive wave moves along the resonator tubeand is reflected back at its closed end. When it reaches the exhaustpipe again it splits and part of it travels towards the exhaust valve.However, it travels together with the negative wave reflected from thesilencer and therefore has no effect on the discharge of exhaust fromthe cylinder. The second positive wave is also reflected back into theresonator pipe, but in the form of a negative wave. This negative waveis reflected back from the closed end of the resonator tube and thenpasses into the exhaust pipe and ultimately arrives at the Reed valve.The lengths of the various pipes are so related that the time of thearrival of the two negative waves at the Reed valve is again such thatthey overlap slightly, whereby the Reed valve is open for a singlerelatively long period of time for each time that the exhaust valveopens.

1. A reciprocating engine (2) including an exhaust system, the exhaustsystem comprising an exhaust pipe (8) which communicates with a silencer(10), whose upstream portion (11) is divergent in the direction of gasflow through it, an oxidising catalyst (12) and an air supply pipe (14)communicating with the exhaust pipe at a position upstream of thecatalyst and silencer, characterised in that the air supply pipe (14)includes a Reed valve (16), that the Reed valve is adapted to open undera pressure differential to permit air to flow into the exhaust pipe (8)and that 3L₂−(2L₁+L₂) is equal to ±0.25 to 0.5 m, wherein L₁ is thedistance from the junction of the exhaust pipe (8) measured from a firstintersection point at which the axes of the exhaust pipe (8) and the airsupply pipe (14) intersect, to a point midway along the length in theflow direction of the said upstream portion (11) of the silencer (10)and L₂ is the length of the air supply pipe (14) from the Reed valve(16) to the said first intersection point.
 2. An engine as claimed inclaim 1 in which 3L₂−(2L₁+L₂) is equal to ±0.35 m to 0.4 m.
 3. Areciprocating engine including an exhaust system (2), the exhaust systemcomprising an exhaust pipe (8) which communicates with a silencer (10),whose upstream portion (11) is divergent in the direction of gas flowthrough it, an oxidising catalyst (12), an air supply pipe (14)communicating with the exhaust pipe at a position upstream of thecatalyst and silencer, and a resonator pipe (20), one end of whichcommunicates with the exhaust pipe (8) at a point between the silencer(10) and the air supply pipe (14) and the other end of which is closed,characterised in that the air supply pipe (14) includes a Reed valve(16), that the Reed valve is adapted to open under a pressuredifferential to permit air to flow into the exhaust pipe (8) and that3L₂−(L₂+2L₃+4L₄) is equal to +0.25 m to +0.45 m or −0.45 m to −0.6 m,wherein L₂ is the length of the air supply pipe (14) from the Reed valve(6) to a first intersection point at which the axes of the exhaust pipe(8) and the air supply pipe (14) intersect, L₃ is the distance betweenthe first intersection point and a second intersection point, at whichthe axes of the resonator pipe (20) and the exhaust pipe (8) intersectand L₄ is the length of the resonator pipe (20) from its closed end tothe second intersection point.
 4. An engine as claimed in claim 3 inwhich 3L₂−(L₂+2L₃+4L₄) is equal to +0.15 m to +0.35 m or −0.35 m to −0.5m.